Flat panel x-ray imaging devices that use charge generator materials such as doped amorphous selenium charge generator layers and directly convert x-rays to electrical charges and thus generate electrical signal related to local x-ray exposure, have been developed in recent years. See, for example U.S. Pat. No. 5,319,206, and Yorker J., Jeromin L., Lee D., Palecki E., Golden K., and Jing Z., “Characterization of a full field mammography detector based on direct x-ray conversion in selenium,” Proc. SPIE 4682, 21–29 (2002). Commercial versions for general radiography and for mammography have been available for more than a year in this country from Hologic, Inc. of Bedford, Mass. (“Hologic”) and Direct Radiography Corporation of Newark, Del. (“DRC”). The DRC imager is used in mammography systems that have been available for more than a year in this country from Lorad Corporation of Danbury, Conn. (“Lorad”). In such direct conversion panels, the charge generator material directly converts x-rays into pairs of electrons and holes and, under an applied electrical field, the holes and electrons migrate to respective electrodes with very little lateral loss to neighboring pixels. Direct conversion is believed to offer better spatial resolution and other advantages over indirect conversion panels, in which x-rays cause scintillation in a material such as cesium iodide and the resulting light energy is detected.
The structure of a direct conversion flat panel imager of the type referred to above is illustrated in principle but not to scale in FIG. 1. It comprises a top electrode 100, a charge barrier layer 102 (typically made of Parylene) separating the top electrode from an amorphous selenium-based charge generator layer 104, an electron blocking layer 106 patterned into a two-dimensional pixel array, a charge collection electrode 108 that also is patterned into a pixel array, a thin-film transistor (“TFT”) array comprising respective transistors 110 coupled to the charge collection electrode and to respective signal storage capacitors 112, a substrate 114 typically made of glass, a gate pulse line 116 that enables (turns ON) the transistors to deliver to charge amplifiers 118 the charges collected at the respective storage capacitors, an a programmable high voltage power supply 120. The illustrated equivalent capacitor circuit for a pixel comprises a capacitor 122 representing capacitance across the charge barrier layer, a capacitor 124 representing capacitance across the charge generator layer, and a capacitor 126 representing capacitance of the charge storage capacitor for the pixel. One of the functions of the charge barrier layer is protection of the thin-film transistors, which can suffer breakdown damage if the charge stored in the charge storage capacitors becomes too high, e.g. when a capacitor stores charges generated at a region of the charge generating layer that receives x-rays that have not been attenuated by the object being imaged. For example, in mammography the corners of the flat panel imager typically are outside the breast outline and can receive much more radiation than the part of the imager under the breast. The charge barrier layer protects such transistors by collecting charges that gradually reduce the electrical field in the appropriate portions of the charge generator layer, and thus reduce the amount of charge that would otherwise collect at the pertinent charge collection capacitors.
The charge barrier layer thus contributes to meeting one of the challenges in flat panel detectors, namely, breakdown protection of the thin-film transistors. Another challenge is ghosting (remnants of one or more previous images) due to the time it takes to dissipate charges collected in the imager from previous x-ray exposures. Various techniques have been developed and used commercially to remove or at least reduce ghosting to an acceptable level. They include charge erasing by exposure to visible light between x-ray exposures and various ways to manipulate the bias potential of electrodes between x-ray exposures. The time needed to attend to ghosting makes it difficult to take images in rapid succession, such as for fluoroscopy or tomosynthesis.
It has been reported that it would be impractical to use a direct conversion panel without a charge barrier layer. Thus, a 1998 paper by well-known researchers in the field of direct conversion panels states that direct metallization of a selenium based detector in theory would allow for rapid imaging but concludes based on experimental data that this gives non-reproducible and unstable results. Polischuk B., Shukri Z., Legros A., and Rougheout H., “Selenium direct converter structure for static and dynamic x-ray detection in medical imaging applications,” SPIE Conference on Physics of Medical Imaging, San Diego, Calif., February 1998, SPIE Vol. 3336, pp. 494–504, states that “In order to develop a selenium based x-ray detector which could operate in real time, i.e. 30 frames per second, a direct metallized selenium structure would be required. It is well established in solid-state theory that metallic electrodes deposited directly onto the free surface of semiconductor layers can behave as Schottky contacts.” The paper then states that “most metals with lower work functions [than selenium] should have built-in potential barriers which could minimize the injection of excess charge from the metal electrode,” but the paper reports that tests showed that “sample-to-sample variability and contact instability were common observations on these samples,” and that “It was therefore concluded that any x-ray detector which relied only on the Schottky contact to limit dark currents would provide non-reproducible and unstable results.” The paper proposes the solution of including a blocking layer between the top electrode and the selenium, and states that “The role of the top blocking layer is to limit the injection of positive charge from the metallic electrode, but allow any x-ray-generated electron to move unimpeded from the selenium layer to the metallic contact.” The authors of the article are from Noranda Advanced Materials of Quebec, Canada, an entity that is believed to have been, at the time, a major developer of flat panel selenium-based x-ray imagers, in addition to DRC.
A number of earlier proposals have dealt with the issue of high voltage protection in flat panel detectors. U.S. Pat. No. 6,353,229, granted to the three authors of the 1998 paper and two other inventors, refers to several such proposals. One is cited at column 1, lines 24–39 and is reported to involve a special dual-gate TFT (thin-film transistor) structure that forms a back channel in the TFT structure if the pixel voltage exceeds a certain potential. See, Zhao W., Law J., Waechner D., Huang Z., and Rowlands J., “Digital radiology using active matrix readout of amorphous selenium detectors with high voltage protection,” 1998 Med Phys 25 (4), pp. 539–549. Another is discussed at column 1, lines 46–57 (U.S. Pat. No. 5,198,673) and is said to involve the use of a second two-terminal protection device resident at each pixel location. The patent also refers, in the section entitled “Description of Prior Art,” to a number of other items of prior art: (1) PCT International Application WO 96/22616 published Jul. 25, 1996; (2) Lee D., Cheung L. K., and Jeromin L., “A new digital detector for projection radiography,” 1995, SPE Vol. 2432, pp. 237–249; (3) U.S. Pat. Nos. 5,598,004 and 5,396,072 (stating that “no mention is made [in those patents] of the high voltage protection of the TFT array”); (4) U.S. Pat. No. 5,528,043 (stating that the patent “does not mention whether high voltage protection of the circuit from the selenium bias is achieved”); (5) U.S. Pat. No. 5,436,101 (stating that “there is no mention of any high voltage protection of any element on the substrate”); and (6) Canadian patent application 2,184,667 published Mar. 4, 1998 and corresponding EP 0 826 983 published the same day (stating that “no indication of how this structure could be used for high voltage protection is given”).
U.S. Pat. No. 6,353,229 proposes to achieve high voltage protection “by setting the high voltage biasing electrode to a negative potential and the TFT “off” gate voltage to a predetermined negative value such that the TFT is essentially non-conductive.” The patent recognizes that “there will always be some TFT leakage” but states that “the negative ‘off’ voltage may be adjusted so as to minimize the same and render the TFT essentially non-conductive.” See column 2, lines 49–61.
Earlier papers and patents are believed to be consistent with the patents and papers cited above. See U.S. Pat. Nos. 5,132,541, 5,184,018, 5,396,072, and 5,942,756, and Zhao W. and Rowlands J. A., “A large area solid-state detector for radiology using amorphous selenium,” SPIE Medical Imaging, Vol. 1. 1651, pp. 134–143, 1992. A more recent U.S. Pat. No. 6,469,312, illustrates in FIG. 2 an electrode 2 on a recording side photoconductive layer 3 containing amorphous selenium as a main component, but has a wavelength conversion layer 1 in front of an electrode layer 2, which serves as a scintillator so that the amorphous selenium later 3 would serve as a light detector.
Each of the patents and papers cited above is hereby incorporated by reference in this patent specification as though fully set forth herein.